Integrated circuits are manufactured using lithographic processes. A lithographic process introduces energy onto selected portions of an energy sensitive photoresist material overlying a substrate. Energy is introduced onto selected portions of the photoresist through openings in a mask substrate interposed between the energy source and the photoresist material. These openings in the mask substrate define the pattern. The pattern is transferred into the photoresist material by the energy that is permitted to pass through the openings in the mask substrate and into the photoresist. Thus, it is an image of the pattern defined by the mask substrate that is transferred into the photoresist material.
After the image is transferred into the photoresist material, the photoresist material is developed to form a pattern. The pattern is then transferred by etching into the substrate underlying the photoresist material. Once the pattern is incorporated into the substrate, it becomes a feature of the integrated circuit. The energy used to expose the photoresist material, the composition of the photoresist material, the thickness of the photoresist material, and many other factors affect the ability of a lithographic process to delineate a feature in a substrate. The smaller the design rule, the more precisely the feature must be delineated. For example, design rules for 0.5 .mu.m are being replaced by design rules of less than 0.5 .mu.m.
Another factor which affects the ability of a process to define features in a substrate is the topography of the substrate surface. Substrate surface topography is either planar or non-planar. Non-planar surfaces are referred to as such because their surfaces are not in a single plane. When a photoresist material is applied over a non-planar substrate, the photoresist layer only approximately conforms to the non-planar substrate surface. As a result, the distance between the photoresist surface and the substrate surface tends to be nonuniform. This nonuniformity can adversely affect the pattern developed in a photoresist material because the depth at which the image is focused in the photoresist will also vary. If the depth of focus varies over the photoresist surface, the features resolved in the photoresist may not satisfy the applicable design rules.
The effects of planar and non-planar surfaces on the lithography process are apparent in the formation of dual damascene structures. Dual damascene structures include line trenches and via trenches. Via trenches are formed in the bottom of the line trenches. FIG. 14 is a cross-sectional schematic view of a prior art dual damascene structure formed on a substrate 500 in an oxide film 530. The dual damascene structure includes via trenches 520 formed in line trench 510.
Current fabrication techniques used to produce a dual damascene structure include forming a first photoresist film on an oxide film to be etched. The first photoresist film is patterned using a line-pattern mask and etched to form the line-pattern in the oxide film. The remains of the first photoresist film are removed and a second photoresist film is formed on the etched oxide film. The second photoresist is patterned using a via-pattern mask and etched to form the via-pattern in the oxide film. Alternatively, the via-pattern and then the line-pattern may be formed in the oxide film. By using this process, the vias and the lines are formed in the oxide film 530 using two, separate photoresist films.
Results achieved using lithography are typically better when the photoresist is applied to a planar surface such as the surface of the oxide film 530 before any etching. When the surface is planar, the photoresist film may be formed with a uniform thickness. As a result, the focal plane has a uniform thickness. In this case, the line-pattern or the via-pattern may be formed in the photoresist film with high fidelity.
In the prior art example above, it is relatively easy to transfer the line-pattern to the photoresist film with high resolution and high fidelity across the surface using state-of-the-art lithographic equipment and materials because the initial surface of the oxide film is planar. Then the line-pattern in the photoresist may be transferred to the substrate with high resolution and high fidelity using reactive ion etching. The surface of the substrate is now non-planar because of this first step. Next, the vias are formed. The photoresist film used to form vias is applied on the non-planar surface. As is shown in FIG. 15, a typical line-pattern consists of large variations in both line widths and space widths. FIG. 15 is a cross-sectional schematic view of a photoresist film 550 for forming the via-pattern on an oxide film 552 after the line-pattern has been etched into the oxide film 552. The line-pattern density varies across the oxide film 552 from low (narrow lines and wide spaces) in the line-pattern-density region 554 to high (wide lines and narrow spaces) in the line-pattern-density region 556.
The via photo resist film 550 is thick in the low line-pattern-density region 554 and thin in the high line-pattern-density region 556. As a result, the focal plane varies because the thickness of the photoresist film 550 varies from region 554 to region 556. Reference lines 558, 560, and 562 illustrate the variations in the thickness of the photoresist film 550. It is more difficult to produce a good via-pattern in the photoresist film 550 using the same state-of-the-art lithographic equipment and materials used to form the line-pattern because the starting surface is non-planar.
A comparison of reference lines 558 and 560 illustrates a second problem caused by forming the via photoresist film 550 on the non-planar oxide film 552. The photoresist film 550 is thicker in a trench than it is in the field surrounding the trench. Usually, the thickness of the photoresist film is selected so that some photoresist material will remain in the field, the area surrounding the via structure 564, after the etch is complete. Typically, the thickness of the photoresist film is also minimized because it is easier to form minimum-sized structures in a thinner photoresist film. In the example shown in FIG. 15, the via structure 564 is in a trench where the photoresist is thick. As a result, an increased exposure dose is necessary to expose the thick photoresist which makes it more difficult to produce minimum-sized vias. The line and via-patterns are therefore more difficult to produce which causes yield loss, the use of more expensive equipment, and more rework. Reversing the process to apply vias first and lines second provides little or no relief from these problems.